In the semiconductor industry it is necessary to measure the widths of lines on photomasks and also in the manufacture of magnetic recording heads it is required to measure the width of the gap in the magnetic circuit. Both these objects form high contrast images which exhibit very little ringing at the edges when viewed using partially coherent illumination. This means that the technique of image shearing may be used to obtain a measure of the width of the line.
In order to measure a linewidth using image shearing, the image is split into two and the two images are displaced relative to each other in a direction perpendicular to the line image until the two images just touch at opposing edges. When this condition is reached the two lines appear to merge into one broad line.
If the image is a dark line on a bright background then this will produce a broad dark line on a bright background with an intensity equal to the mean of the bright and dark areas. If the displacement of the two images is too small then a dark line will appear where the two images meet. Similarly, if the displacement between the two images is too great, then a bright line will appear where the two line images meet.
In conventional manual measurement systems a shearing mechanism is adjusted manually until the two line images merge into one with neither a bright nor dark line between them. The position of the shearing mechanism is recorded and used as a zero reference. The images are then displaced by an equal and opposite amount and the shearing mechanism is adjusted until the two images merge into one. The difference between the new position of the shearing mechanism and the previous reference zero position is proportional to a measure of twice the width of the line. Thus a linewidth measurement may be computed.
The present invention has been developed using a novel technique for detecting the degree of misalignment of two opposing edges in a sheared image and using this to automate the measurement of a linewidth in an optical image. A signal is generated to indicate the degree of misalignment and this may be fed back to the shearing mechanism and used to set the shearing to the correct edge setting.